Fault current limiters are used to provide protection against current surges, for example in a power transmission network. Superconducting Fault Current Limiters (SCFCL) are a class of devices that operate at a cryogenic temperature and are typically used in electrical transmission or distribution lines that are subjected to high voltages and high currents. In a resistive SCFCL, the current passes through the superconductor component of the SCFCL such that when a high fault current begins, the superconductor quenches in that it becomes a normal conductor and the resistance rises sharply and quickly.
In particular, the core of a SCFCL device may consist of several superconducting elements that are interconnected in series and parallel using non-superconducting connectors, which may dissipate power and increase cryogenics thermal load. In a normal operating mode, the SCFL device is cooled to cryogenic temperatures in order for the superconducting elements, such as tapes, to enter the superconducting state. Under a non-fault state, current passes through the superconducting tapes and into connector regions that exhibit normal-state (that is, non-superconducting) current conduction, which may be composed of conventional metals. When a current surge takes place along a transmission line, the current may enter the SCFCL at which point it travels through the superconducting elements. If the current surge exceeds a critical value in the superconducting tapes, the superconducting material may transform into a normal conductor (i.e. quench). Once in the normal conducting state, the superconductor material acquires a resistance to current which may limit the current conducted through the SCFCL to acceptable levels, thereby regulating the current conducted along the transmission lines.
SCFCLs that are under active development include, among others, systems using magnesium diboride wire, Yttrium Barium Copper Oxide (YBCO) tape, or Bismuth Strontium Calcium Copper Oxide (BSSCO) materials, which are cooled to below their respective superconducting transition temperatures (Tc) in order to function as designed. YBCO and BSSCO-based devices are attractive because the Tcin typical commercial materials is in the range of 90°-105° K, allowing SCFCL devices to operate using relatively inexpensive liquid nitrogen or boiling nitrogen cooling.
In known SCFCL devices that use tape-type structures, a superconducting layer may be laminated with other non-superconducting layers that include metal cladding. The composite tape may be joined to other tapes to form an SCFCL device. In particular, a series of superconducting tapes can be coupled using electrically conductive connectors having normal conductivity, such as metallic elements. For example, superconducting tapes may be soldered to metal connectors that form interconnections between the tapes. However, this type of configuration is prone to developing hot spots caused by highly non-uniform current distributions, and to connection power losses.
Moreover, significant power losses may occur when current passes between superconducting and non-superconducting regions. In some configurations, an SCFCL design may contain many hundreds of connection points between superconducting and non-superconducting elements in which hundreds of watts of steady state power are lost. Accordingly, it will be apparent that improvements are desirable over known SCFCL systems.